Nanoscale serrated framboidal pyrite documents pulsed bottom‐water oxygenation in the early Cambrian anoxic ocean

ABSTRACT Sedimentary framboidal pyrite (FeS 2 ) is an important material for reconstructing paleo‐marine redox environments, as its morphological features preserve information of syndepositional to early diagenetic oxidation processes in ancient marine sediments. Accurately constraining the timing and intermediate products of pyrite oxidation as well as determining whether pyrite retains detectable reactive signatures under suboxic conditions are critical. In this study, we focus on a continuous, ~100‐m‐long black shale core interval of the lower Cambrian Shuijingtuo (SJT) Formation (Core LX03 in the Yichang area, western Hubei, China). We investigated nanoscale‐ to atomic‐scale oxidation features of framboidal pyrite associated with redox changes in the early Cambrian ocean via Ar‐ion polished scanning electron microscopy (SEM) and spherical aberration‐corrected transmission electron microscopy (ACTEM). Our results revealed that framboids with irregularly serrated margins occur in mm‐ to cm‐thick layers in the upper SJT Formation. The serrated margins show variable structure, some being enclosed in a ~5‐nm‐thick amorphous passivation layer composed primarily of Fe and O. High‐resolution STEM analysis further revealed that the oxidation byproducts were dominated by marcasite (FeS 2 ), ferrous sulfate (FeSO 4 ) and iron sulfate hydrate (FeSO 4 ·nH 2 O), which occur as oxidised phases indicative of localised oxidative dissolution processes. These findings demonstrate that the serrated margins of framboidal pyrite microcrystals formed via staged oxidation driven by progressive increases in the redox potential (Eh) of Cambrian oceanic bottomwaters. The stratigraphic association of these microtextures with exceptionally preserved benthic fossils suggests that pyrite morphological characteristics and crystallographic architectures record syngenetic to early diagenetic redox fluctuations. Thus, the serrated framboidal pyrite microcrystals documented in this study provide direct mineralogical evidence for pulsed bottom‐water oxygenation events, explaining unusual occurrences of benthic animals in widely anoxic early Cambrian oceans.